High index optical glass

ABSTRACT

An optical glass having a high index of refraction, high dispersion and low absorption in the visible region of the spectrum produced from melts comprising lead oxide (PbO), tellurium dioxide (TeO2), germanium dioxide (GeO2), sodium oxide (Na2O) and silicon dioxide (SiO2).

United States Patent Wylot et a1.

HIGH INDEX OPTICAL GLASS Inventors: James Matthew Wylot; Edgar Joseph Greco, both of Rochester, NY.

Eastman Kodak Company, Rochester, NY.

Filed: Sept. 5, 1972 Appl. Noi: 286,499

Published under the Trial Voluntary Protest Program on January 28, 1975 as document no.

Assignee:

US. Cl 106/47 Q; 106/53 Int. Cl. C03C 3/12 Field of Search 106/47 Q, 47 R, 53

References Cited UNITED STATES PATENTS 11/1926 Dennis 106/47 R Oct. 21, 1975 2,673,809 3/1954 Weissenberg et a1. 106/47 R FOREIGN PATENTS OR APPLICATIONS 736,073 8/1955 United Kingdom 106/47 O OTHER PUBLICATIONS Dennis et al., Germanium Glasses," J. Soc. Glass Tech. 9, p. 184, (1925).

Primary Examiner-Anthony Skapars Assistant ExaminerThomas A. Waltz Attorney, Agent, or FirmR. F. Brothers [57] ABSTRACT 1 Claim, No Drawings HIGH INDEX OPTICAL GLASS CROSS REFERENCE TO RELATED APPLICATIONS Reference is made to commonly assigned copending U.S. Pat. application Ser. No. 286,498, filed Sept. 5, 1972, in the names of Edgar Joseph Greco and James Matthew Wylot.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to optical glasses and in particular to optical glasses having a high index of refraction, high dispersion and low absorption in the visible region of the spectrum.

2. Description of the Prior Art It is well known in the glass and lens making arts that optical glasses having high indices of refraction, high dispersion and low absorption in the visible region of the spectrum would be of great utility in photographic systems. A high index of refraction permits reduction of the curvature required in a lens element to produce a desired optical performance, thereby making lens production simpler and less expensive and permitting production of otherwise impractical lenses; Low absorption in the visible region of the spectrum, i.e., low color, is desirable in a photographic lens since it permits satisfactory exposure with lower ambient illumination and because distortion of scene color by the lens system is minimized. High dispersion permits greater correction of chromatic aberrations, thereby permitting design and manufacture of lens systems which would otherwise be unusable in photographic apparatus.

Flint glasses which are characterized by a high index of refraction have long been known and studied by glass technologists. However, such high index flint glasses have been characterized by a yellow color which makes them unsatisfactory for use in photographic apparatus. Recent developments in lead-tellurium flint glasses have resulted in optical glasses which are characterized by a high index of refraction as well as low absorption in the visible region of the spectrum. Specific examples of these glasses are disclosed in the referenced related patent application, U.S. Ser. No. 286,498, filed Sept. 5, 1972, in the names of Greco and Wylot.

The addition of certain alkali oxides, in particular sodium oxide (Na O), to flint glasses is known to usually increase the dispersion of the glass, i.e., decrease the Abbe number. However, alkali containing flint glasses generally exhibit increased color as well as decreased chemical durability, both undesirable qualities in glasses for use in photographic apparatus. We have discovered that the dispersion of some of the flint glasses disclosed in the referenced related patent application may be increased by the addition of sodium oxide (Na O) without a significant increase in color or decrease in chemical durability. In particular, we have discovered that up to percent by weight of Na O may be added to glasses in the lead oxide-tellurium oxide-germanium oxide system without significant color increase, while up to 7 percent by weight of Na O may be added without a significant decrease in chemical durability.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an optical glass having high dispersion.

Another object is to provide such an optical glass which is durable.

Another object is to provide such an optical glass having a high index of refraction.

Still another object is to provide such an optical glass having low absorption in the visible region of the spectrum.

It has been found that these and other objects are accomplished according to the present invention by the addition of sodium oxide (Na O) to certain flint glasses, to provide optical glass consisting of components having the following weight percents:

Component Weight (Percent) Lead oxide (PbO) 64.7 Tellurium dioxide (TeO 9.7 Germanium dioxide (GeO 7.6 Sodium oxide (M1 0) 5 Silicon dioxide (SiO,) l3

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 7 TABLE I No. PbO TeO GeO; Na,0 n V D 57.7 9.7 27.6 5 1.962 18.2 135E 62.7 9.7 22.6 5 2.001 16.72 135H 57.0 8.8 25.1 9.1 1.920 17.5 1351.. 56.0 8.6 24.7 10.7 1350 67.7 9.7 12.6 10 1350 59.7 9.2 g 26.3 4.8 1.977 17.8

Each of these glasses is characterized by high dispersion, all having an Abbe number lying between 16.0 and 22.0, due apparently to the addition of sodium oxide (Na O) to the melts. In addition, these glasses are characterized by high indices of refraction, all having an index in excess of 1.90. Moreover, all of these glass exhibit low absorption of light in the visible region of the spectrum. Finally, no measurable deterioration of chemical durability has been observed in those glasses in which the amount of sodium oxide (Na O) is limited to 7 percent or less by weight.

It has been found that up to 18 percent silicon dioxide (SiO may be substituted for germanium dioxide (GeO in these glasses, thereby resulting in a less expensive glass composition. However, increasing the amount of silicon dioxide above 18 percent results in increased color and decreased durability. Accordingly, Table II illustrates examples of glass compositions in which silicon dioxide has been added to the system in partial substitution for germanium dioxide (GeO 800 C and stirring was continued for 30 minutes. The melt was then cast onto a cast iron table whose temper- These glasses were fired in gold crucibles at a temperature in the range of 800950 C.

TABLE ll N0. PbO T60 060 N320 $10 11,, v,,

135g 62.7 9.7 17.6 5 1.961 17.85 1351 62.7 9.7 7.6 5 15 1.884 20.6 135K 62.7 12.7 7.6 5 12 1.90 135M 62.7 15.7 7.6 5 9 1.959 18.04 135N 68.7 9.7 7.6 5 9 1.968 17.24 135v 62.7 9.7 12.6 5 1.919 19.5 1351-1 64.7 7.7 7.6 5 1.891 20.32 1341-2 64.7 7.7 5.6 7 15 1.86 13518 64.7 7.7 7.6 7 13 1.894 19.22 13514 59.7 9.2 21.3 4.8 5 1.942 19.30 1351 5 66.7 7.7 7.6 7 11 1.918 18.08 13516 64.7 9.7 7.6 7 11 1.919 18.7

66.7 7.7 5.6 7 13 1.89 mm 1351-8 64.7 9.7 5.6 7 13 1.898 19.34 13519 62.7 11.7 5.6 7 13 1.88 1351-10 62.7 11.7 7.6 7 11 1.89 1351-11 64.7 9.7 7.6 5 13 1.912 1351-1 1A 61.6 9.2 6.8 5.2 17.2 1.863 1351-12 64.7 9.7 5.6 5 15 1.88

Each of these glasses is also characterized by high (Na O) is limited to 7 percent or less by weight. Component weighrwercent) By way of specific example of an optical glass pro- Pbo 6L6 duced according to this invention, a sample of melt 2 9.2 135I-1 1A as illustrated in Table 11 was prepared from fg 3g the following components: N320 5.2

Component Weight (gms) This 1nvent1on has been descrlbed in detail wlthpar- 4 PbO 1294 TeQz 193 t1cular reference to the preferred embod1ments 2 142 thereof, but it will be understood that variations and. 1 fig 333 modifications can be effected within the spirit and The TeO and Na CO were placed in a twin-shell blender and blended for a period of thirty minutes. as

were the PbO, Ge0 and SiO The former mixture was 5 then placed ina gold crucible and fired at a temperature of 900 C for 1 hour. The remaining mixture of ature was maintained at 150 C and the glass was subse quently annealed at a temperature of 290 C. The resul- 1 tant glass as illustrated in Table II has an index of re-- fraction of 1.863 and an Abbe number of21.60 and the following composition:

scope of the invention.

We claim:

1. Optical glass having an index of refraction of approximately 1.9l-2 and an Abbe number of approximately 19.6 and consistingof components having the following weight percents:

PbO, GeO and $10 was then added to the crucible Component Weight Percent and firing was continued for an additional 40 minutes Lead Oxide (Pbo) 64] at 900 C. Stirring was then begun w1th a three-bladed, 5 gelluriurn dig id dfleco g) 3.; three-tiered gold stirrer at 200 rpm. Stirring was con ig j e z) tmued for 10 mmutes at a temperature of 900 C. St1r- Silicon dioxide ($10,) 13.

ring was then halted for 30 minutes and then stirring was again commenced and continued for 15 minutes at 900 C. The melt temperature was then reduced to 

1. OPTICAL GLASS HAVING AN INDEX OF REFRACTION OF APPROXIMATELY 1.912 AND AN ABBE NUMBER OF APPROXIMATELY 19.6 AND CONSISTING OF COMPONENTS HAVING THE FOLLOWING WEIGHT PERCENTS: 